Thermally bonded nonwoven webs are well known in the art (Wendt, Industrial and Engineering Chemistry Volume 48, No. 8 (1965) pages 1342; U.S. Pat. Nos. 3,978,185, 3,795,571; 3,811,957). Stretching of nonwoven webs is described in U.S. Pat. Nos. 3,772,417, 4,048,364, 4,223,059, 3,949,127, 4,276,336, 5,296,289, 4,443,513 and EP 0 882 147. However, none of these disclosures relates to the causal connection of stretching of a nonwoven web and imparting elastic properties.
Thermally bonded nonwoven webs are conventionally used for the mass production of disposable sanitary protection products such as adult and infant diapers or sanitary napkins, medical products such as masks, operating gowns, head covers or operating drapes; protective work-wear such as coveralls, head covers and masks; and personal use items such as underwear. A major deficiency of nonwoven webs is their lack of elasticity or stretch and conformability. Since conventional thermally bonded nonwoven webs do not have sufficient elastic properties, products containing such nonwoven webs which require elastic properties conventionally further contain latex bands for fastening and fitting. However, proper adjustment of latex straps is difficult to achieve whereby a fit is usually observed which is either too loose or too tight. Moreover, latex straps are allergenic and irritating to the skin to some degree. Additionally, the use of latex and rubber components in huge volume for disposable products has raised serious environmental concerns in view of toxic waste generation such as dioxins and other harmful emissions in the waste incineration process.
Attempts were made in the prior art to provide nonwoven webs having elastic properties. In one approach, elastomers are incorporated into nonwoven webs as films, bands, or threads of natural or synthetic rubber whereby full-web elasticity in two directions is achieved. However, nonwoven webs based on elastomers lack dimensional stability in at least one direction whereby it is difficult to handle such webs in automated manufacturing processes. Moreover, nonwoven webs based on elastomeric fibers are expensive. Therefore, the use of elastomeric fibers poses inherent problems which render them unsuitable for the mass production of disposable products.
An alternative approach for imparting elasticity to a nonwoven web relates to the socalled thermo-mechanical treatments. Thermo-mechanical treatments for imparting elasticity to a nonwoven web are described in U.S. Pat. No. 5,244,482 and EP 0 844 323. Accordingly, a thermally bonded nonwoven precursor web is subjected to a stretching treatment at an elevated temperature in one direction (machine direction) whereby the width of the precursor web shrinks in perpendicular direction (cross direction) resulting in a certain elasticity in cross direction while maintaining non-elastic properties in machine direction. The anisotropic elasticity combining dimensional stability in machine direction and elastic properties in the cross direction facilitates the use of such webs in automated manufacturing processes.
U.S. Pat. No. 5,244,482 disclosed a process for the preparation of a filter material, wherein very high strain rates of at least 2500%/min are used to laterally consolidate the precursor web with resultant width of less than 80% of the precursor. The very high strain rates are shown to change the morphology of the nonwoven web, reduce the pore size and narrow the pore size distribution. Although a degree of elasticity is created, the elastic modulus is low (70% recovery at 50% elongation, 40% recovery at 100% elongation). We already learn a low draw ratio will not make a high stretchy resultant web. The required strain rates mean in a continuous process, that a high draw ratio with a high processing speed of from 1000 to 4000 m/min are unlikely to be achieved in practice. Moreover, the resultant fabrics is stiff and with specially selected precursors whereby mass production of disposable products based on the material of U.S. Pat. No. 5,244,482 is not possible.
EP 0 844 323 discloses a process wherein a nonwoven web is stretched under low strain rates of from 350 to 950%/min and carefully controlled thermal process conditions for creating a degree of elasticity (85% recovery at 50% elongation) within the precursor web. However, the degree of elasticity of the resultant webs turned out to be still insufficient for meeting the standards required for commercially successful applications. Moreover, although the process of EP 0 844 323 may be carried out in a continuous mode, the maximum process speed attainable is well below 100 m/min whereby mass production cannot be considered economical.